A Mn^III₂Fe^II chain with a [–Mn^III–(O_Ph)₂–Mn^III–ON–Fe^II–NO–] repeat unit was synthesized via the assembling reaction of Mn^III salen-type dimers and Fe^II pyridyloximate complexes, where the –(O_Ph)– and –ON– bridges represent a biphenolate bridge and an oximate bridge, respectively. The bulky counter anions, BPh₄⁻, which surround the chain to form a zeolite-like brick wall, successfully isolate the chains from a magnetic point of view. This compound is isostructural with Mn₂Ni–BPh₄, which is an SCM with S_T = 3 (H. Miyasaka, A. Saitoh, M. Yamashita and R. Clérac, Dalton Trans., 2008, 2422). Because the Fe^II unit is diamagnetic, the Mn^III dimer, which has the potential to be a single-molecule magnet (SMM), is nearly magnetically isolated, although a weak antiferromagnetic interaction with J/k_B ≈ −0.1 K is perturbed between the Mn^III dimers with S_T = 4 via the Fe^II unit. Ac susceptibility data shows that two types of relaxation of the magnetization are present, which are attributed to SMM and chain relaxations.